New Chronologic and Geomorphic Analyses of Debris Flows on Mount Rainier, Washington

By Ian Delaney

Whitman College, Walla Walla WA

Debris flows on Mount Rainier are thought to be increasing in frequency and magnitude in recent years, possibly due to retreating glaciers. These debris flows are caused by precipitation events or glacial outburst floods. Furthermore, as glaciers recede they leave steep unstable slopes of till providing the material needed for mass-wasting events. Little is known about the more distant history and downstream effects of these events. Field observations and/or aerial photograph of the Carbon River, Tahoma Creek, and White River drainages provide evidence of historic debris flow activity. The width and gradient of the channel on reaches of streams affected by debris flows is compared with reaches not affected by debris flows. In all reaches with field evidence of debris flows the average channel width is greater than in places not directly affected by debris flows. Furthermore, trends in width relate to the condition of the glacier. The stable Carbon Glacier with lots of drift at the terminus feeds the Carbon River, which has a gentle gradient and whose width remains relatively consistent over the period from 1952 to 2006. Conversely, the rapidly retreating Tahoma and South Tahoma Glaciers have lots of stagnant ice and feed the steep Tahoma Creek. Tahoma Creek’s width greatly increased, especially in the zone affect by debris flow activity. The rock-covered, yet relatively stable Emmons Glacier feeds White River, which shows relatively little change in channel width.